1. Field of the Invention
The present invention relates to a rear-wheel steering system of a motor vehicle.
2. Description of the Related Art
A rear-wheel steering system of a motor vehicle is disclosed in, for example, Japanese Patent Application Laid-Open 7-215226. This rear-wheel steering system is provided with a nut member contained in a housing and connected to an electric motor through a planetary gear unit, and a rod having an external thread portion in mesh with an internal thread portion of the nut member. In such a system, the rod is laterally moved across a width of the vehicle by rotation of the electric motor so as to steer rear wheels connected thereto.
Another rear-wheel steering system having similar structural features as that of the above-described system is disclosed in Japanese Patent Application Laid-Open 7-285450.
Referring to FIG. 3, the generally employed rear-wheel steering system as described above has a nut member 120 for converting a rotational motion of an electric motor into an axial straight motion of a rod 110. The nut member 120 has an internal thread portion 120a, which is engaged with an external thread portion 110a of the rod 110. In the course of machining those thread portions 120a and 110a, however, each dimension thereof is likely to vary depending on the individual machining process condition. As a result, looseness between the internal thread portion 120a and the external thread portion 110a is unavoidably generated. Such looseness may cause the wheel of the running vehicle to slightly vibrate, leading to noise from the thread portions in mesh with each other.
In order to eliminate the looseness between the internal and external thread portions, the precision of machining the thread portions has to be improved. However, this may raise production costs substantially.